It has been conventionally known that fillers, for example, short fibers such as aramid, microfibrillated plant fibers such as cellulose fibers, or crystalline polymers such as syndiotactic polybutadiene can be incorporated into rubber compositions to reinforce the rubber compositions to improve the modulus (complex modulus). However, fillers are highly self-aggregative and poorly compatible with rubber components in many cases. For example, when microfibrillated plant fibers are introduced into and mixed with rubber latex, unfortunately, about 20% of the microfibrillated plant fibers are not incorporated into the rubber component but remain in the solution.
In masterbatches prepared by mixing rubber latex with fillers, the fillers tend to easily form aggregates. For example, tires formed from such masterbatches may suffer rapid wear, cracks, chipping, or separation between layers due to the formed aggregates, thereby possibly resulting in air leakage or loss of handling stability. Therefore, there is a need to enhance the dispersibility of fillers in rubber in masterbatches.
In order to enhance the dispersibility of fillers in rubber in masterbatches to improve rubber physical properties, conventional methods for preparing a masterbatch include mixing rubber latex with fillers and adjusting the pH of the mixture. Other methods are disclosed, including, for example: methods of mixing a carbon black-containing slurry solution having a predetermined zeta potential with a rubber latex solution followed by coagulation and drying to produce a wet masterbatch (see, for example, Patent Literature 1); methods of breaking the amide bonds in natural rubber latex and mixing the resulting latex with a slurry solution of inorganic filler to produce a natural rubber masterbatch (see, for example, Patent Literature 2); methods of mixing a slurry of inorganic particles with a polymer latex having a surface potential of a sign opposite to that of the slurry of inorganic particles to produce a polymer composite (see, for example, Patent Literature 3); methods of mixing together aqueous dispersions of single components in which the particles have a surface charge of the same sign and a predetermined zeta potential, and the ratio between the zeta potentials of the particles of each dispersion is within a predetermined range, followed by coagulating the mixed dispersion thus obtained (see, for example, Patent Literature 4); methods of removing water from an aqueous dispersion which contains rubber latex and a cellulose nanofiber having a predetermined average fiber width, and has a predetermined solids concentration to produce a rubber masterbatch (see, for example, Patent Literature 5); and methods of removing water from a mixture which contains a resin emulsion and a fine cellulose fiber having a predetermined average fiber width, and has a predetermined solids concentration to produce a composite (see, for example, Patent Literature 6).